Implementation of Wavelength Division Multiplexing (WDM) and Dense Wavelength Division Multiplexing (DWDM) technology in fiber communications systems has led to significant improvements in data transmission rates and available bandwidth. Etalons have been proven useful for many WDM and DWDM technologies, and for example, are found in interleavers/deinterleavers, wavelength lockers, spectrum analysers, and optical filters.
In general, an etalon is formed from two partially transmitting mirrors, or surfaces, separated by a predetermined gap that forms a cavity. Referring to FIGS. 1a, 1b, an etalon is shown at resonance transmitting a series of equally spaced wavelengths, λ1-n. In particular, the etalon has a periodic response to a multi-wavelength input signal, according to the following equation2d·n·cos θ=mλ  (1)where d is the width of the gap, n is the index of refraction of the medium in the cavity, θ is the angle of incidence of the input beam, and m is the mode number or order of interference. When the cavity medium is air or vacuum, the etalon is referred to as an air-spaced etalon. When the cavity medium is a transmissive solid, such as glass, the etalon is called a solid etalon.
In air spaced etalons, each mirror is typically a thin partially reflective coating deposited on an optically transparent substrate. The mirrors are arranged with the reflective surfaces facing one another, while a spacer disposed out of the optical path, provides the predetermined gap therebetween. For example, air-spaced etalons have been manufactured from a pair of opposing multi-layer thin film filters and separated by a fused silica spacer. In solid etalons, the reflective coatings are deposited directly on opposing ends of a relatively thick, optically transparent substrate that serves as the spacer and the cavity medium. In these cases, the width of the gap is equal to the width of the substrate.
There are a number of disadvantages associated with most air-spaced and solid etalons found in the prior art. A first disadvantage of most prior art etalons is that the substrate, i.e., the filter substrate in air-spaced etalons or the spacer in solid etalons, limits the optical performance of the etalon. For example, the substrate is generally associated with a specific transmission window and a small amount of loss that is dependent upon the composition and thickness of the substrate. Furthermore, since the composition and thickness of the substrate must be chosen for a specific transmission window, and such that it is compatible with the deposition process, it is difficult to engineer the substrate for other properties, such as thermal stability. U.S. Pat. No. 5,156,720 to Rosenfeld entitled PROCESS FOR PRODUCING RELEASED VAPOUR DEPOSITED FILMS AND PRODUCT PRODUCED THEREBY, U.S. Pat. No. 5,225,926 to Cuomo et al. entitled DURABLE OPTICAL ELEMENTS FABRICATED FROM FREE STANDING POLYCRYSTALLINE DIAMOND AND NON-HYDROGENATED AMORPHOUS DIAMOND LIKE CARBON (DLC) THIN FILMS, and U.S. Pat. No. 6,103,305 to Friedmann et al. entitled METHOD OF FORMING A STRESS RELIEVED AMORPHOUS TETRAHEDRALLY-COORDINATED CARBON FILM, incorporated herein by reference, each disclose free-standing thin films that obviate some disadvantages of substrates.
A second disadvantage of prior art etalons relates to thermal instability, as alluded to above. According to Eq. 1 the spectral response of an etalon is dependent on the width of the gap and the refractive index of the cavity medium. If the etalon spacer is fabricated from a material having a large coefficient of thermal expansion, an increase in temperature can increase the gap width, and as a result, shift the spectral response. Advantageously, spacers made with material having a low coefficient of thermal expansion can make single air-spaced etalons substantially temperature insensitive. However, this is not typically the case for solid etalons, where the change in refractive index of the spacer with temperature will also affect the optical path length of the cavity. For example, Corning's ULE™ and Schott's Zerodur™ both have an approximately zero coefficient of thermal expansion and exhibit a positive change in index of refraction with increasing temperature. Various attempts to create athermal etalons have been proposed, as for example, in U.S. Pat. No. 5,384,877 to Stone entitled PASSIVE TEMPERATURE-INSENSITIVE FABRY-PEROT ETALONS, U.S. Pat. No. 5,375,181 to Miller et al. entitled TEMPERATURE COMPENSATED FIBRE FABRY-PEROT FILTERS, U.S. Pat. No. 6,215,802 to Lunt entitled THERMALLY STABLE AIR-GAP ETALON FOR DENSE WAVELENGTH-DIVISION MULTIPLEXING APPLICATIONS, and U.S. Pat. No. 6,005,995 to Chen et al. entitled FREQUENCY SORTER, AND FREQUENCY LOCKER FOR MONITORING FREQUENCY SHIFT OF RADIATION SOURCE, all incorporated herein by reference.
A third disadvantage of prior art etalons, that is also a consequence of their thermal instability, relates to the difficulty in producing high performance multi-cavity etalons. Multi-cavity etalons, which have two or more sequential cavities, show great potential for producing complex spectral responses. For example, it is known that a multi-cavity etalon can exhibit a wider and squarer spectral response than a single cavity etalon. See, for example, the paper referenced as J. Stone, L. W. Stulz, A. A. M. Saleh, “Three-mirror fibre Fabry-Perot filters of optimal design, Electronics Letters, Vol. 26, No. 14, July 1990. However, to date, it has not been feasible to create an athermal multi-cavity etalon without tuning the etalon and/or providing a thermally stable environment, since the substrates, which are part of the mirrors in air-spaced etalons and serve as the cavity in solid etalons, exhibit a significant change of refractive index with temperature and typically have a moderate to high coefficient of thermal expansion.
It is an object of this invention to provide a thermally stable multi-cavity etalon.
It is another object of this invention is to provide a thermally stable etalon.
It is another object of this invention is to provide an etalon fabricated from self-supporting filters.